exchemist

I don't think even the Russians would see military potential in this ice engine of yours! But I was thinking about it this afternoon. It could be interesting to distill it down to its simplest. You have a vertical cylinder with water in it, a piston and a ratchet so that when it freezes it lifts a weight and when it melts, the weight can't fall back down. You alternately connect the cylinder to a warm reservoir at say 20C and a cold reservoir at say -20C. The warm reservoir provides heat to melt the ice and the cold reservoir freezes it again. What happens? You start with water, connect to the cold reservoir and freeze the water. Heat flows into it, corresponding to Latent Heat of Fusion minus the work done in lifting the weight. Then you connect to the warm reservoir. Heat flows from it to the cylinder to melt the ice, this time the full Latent Heat of fusion, as the water is no longer under pressure, thanks to the ratchet. So you have a heat flow from warm to cold, with some of the heat being converted to mechanical work. It's a normal heat engine, really, isn't it?

Ah, so it was Tesla, again. Here we go.... And he was looking for a perpetual motion machine of the second kind, that is to say one operating with only a heat source and no heat sink. Fine, we all know how that ends. But I see you are personally quite serious about this: https://experiment.com/projects/hohohltuqpivlpspyewk/methods Now I understand why you talk of getting banned, men in black etc.

You know the pressure change is not significant from the tension or lack of it in your cheeks. But I looked it up and, in pursed lip breathing exercises for medical conditions, patients typically generate a pressure of the order of 15cm H2O, i.e. ~10mmHg. I leave it to you to calculate the temperature drop from adiabatic expansion by this amount, if you think it is significant.

It seems to me @Bufofrog has got it pretty well spot-on. Air entrainment will account for the faster stream of air being a bit cooler. There is negligible pressure drop across pursed lips, so adiabatic cooling won't be relevant. Cooling by evaporation - accelerated by rapid removal of evaporated vapour - will be very important in the soup/tea case. A cup of tea drunk outside cools a lot faster than indoors at the same temperature, if there is a bit of a breeze.

I think that has to be right. In science we make models of the "real" physical world, to predict what we should expect to observe. There comes a point, certainly in QM as I remember it, at which the mathematics is the model. Is it real? The honest answer has to be that we don't know. All our models are subject to change and further refinement, so we can never say they are 100% real, but only approximations to reality.

Are we all barking up the wrong tree because of your initial choice of term for the machine you have mind? Are you perhaps thinking of a hydraulic intensifier: https://en.wikipedia.org/wiki/Hydraulic_intensifier ?

Haha, I did wonder if you were a closet free energy crank, what with all your talk of being banned and everything. Can you refer me to a link to this 1900 fellow? Clearly he wasn't quite as smart as all that - or maybe he wanted to pull a few people's legs. Like you perhaps.

Well, you still need to induce the water to freeze, of course. And the freezing point will be depressed by the pressure, because the pressure will shift the point of equilibrium between water and ice. So you will need a colder fridge.

Er, not quite. Chemical energy is converted into work as well as heat, as the bonds form during crystallisation. But it would indeed mean less heat for the refrigeration to remove, yes.

It's because you don't understand the science and make statements that are not credible, I expect. It looks to me as if we need to talk a bit about enthalpy, H. Enthalpy, H=U +PV, where U is the internal energy of a system and PV is the work done done on, or by, the system due to its expansion or contraction under any prevailing pressure during changes to the system. In chemistry we generally work with enthalpy because most chemical reactions are carried out at atmospheric pressure. So any changes in volume during a reaction will either push back the atmosphere (if the reacting system expands) or get pushed on by the atmosphere (if it contracts). These effects alter the total amount of energy measured as coming out or going into the reaction system. So, in the case of ice freezing, Latent Heat of Fusion comes out of the water as it freezes. If the system is open to the atmosphere, the amount of heat that comes out will be a bit less than if the water was being frozen under vacuum. That's because some of the internal energy released, as the bonds in the ice crystal form, goes into pushing back the atmosphere as the ice expands. In a vacuum there is no work done, so all the internal energy from bond formation comes out as Latent Heat. If you now put the water in a cylinder underneath a huge weight and freeze it, the heat that comes out during freezing will be less still, because more of the internal energy in the forming bonds goes into pushing up the weight. So in summary, there is a fixed amount of internal energy, per gram - per molecule, in fact - that is released when ice forms. How much comes out as Latent Heat depends on the PV work the water has do as it freezes. There is no free lunch here.

No.

I can't do this in these silly units, but if the pump can generate a certain pressure you just need to make sure your system does not require a greater pressure to lift whatever it is you want to lift. Force is pressure x area, so with enough area you can produce any force you want. The catch of course is the rate of lifting, in terms of vertical distance lifted, goes down as the area goes up, because the pump can only introduce fluid at a fixed rate, so the bigger the area the slower it lifts.

No. I'm getting fed up with you now. Reported.

Well in that case you won't get any "explosion", will you? The piston will gently move out by 9% as the water freezes and, er, that's it. It can be made to do the same amount of work as your earlier ice bomb explosion, of course, but just in a less dramatic way. Correction: Sorry I see you propose something that makes it go ping when the pressure has built up. Not sure what advantage that has, but fair enough, it will go ping, then.

OK, but it stretches a bit. What happens is you stretch it beyond the elastic limit, reach the yield point and then it fails.

There are a lot of anti-vaxxers and nutjobs about at the moment, as you may be aware, that like to cast doubt on the safety and efficacy of vaccination. It looks as though some of the people here have got burnt by this in the past and may have thought you could be one of them, rather than just someone who is not well informed. Risks from the vaccines are small compared to the risks from the disease, even for younger people. Aside from the risk of death or emergency hospitalisation if you are fat, there's a lot of long Covid about. I have a nephew who is very fit (got a half blue at Oxford, rowing for the Lightweights), whose sense of taste has been permanently altered by the virus. I lost my sense of taste and smell totally for a fortnight, which became a bit frightening, though luckily it has come back. The risk of blot clots with the Oxford/AstraZeneca one seems to be about one in half a million: about a fiftieth of the annual risk of blood clots run by a woman on the pill. Nevertheless for those under 40, it is recommended to get one of the other vaccines if possible, for preference, just to avoid even this small risk. As others have pointed out, by getting vaccinated you also help reduce the incidence of Covid in the population as a whole, which reduces the chance of more, nastier, mutations coming along and setting us all back to square one. So yes, get vaccinated. I had my second shot last week (AstraZeneca).

I'm not sure I follow this. It looks as if you envisage a closed container that stretches and then suddenly bursts from the pressure of the ice and that this moves a piston. Depending on what load the piston is connected to, I suppose with a light load it might fly away from contact with the ice, leaving a near-vacuum behind. It won't be quite a vacuum as there will be the vapour pressure of water or ice at 0C present - about 4.5mmHg, so not much, admittedly. A vacuum won't cause the ice to melt. Many comets are made of ice. So there won't be any boiling. Why is this piston "gigantic", suddenly? Are you smoking exotic cheroots? 😀

I think we should leave urinary infections out of this. 😆

Strictly, yes of course, but the way this problem has been posed, you don't actually need the sign to get the right answers.

Eh? Lifting weights expands nothing. Work done in expansion is PdV. Think of it this way: if you have an expanding fluid pushing a piston, the work done is the force, F, on the piston (pressure x surface area) multiplied by the distance, d, the piston travels - which is volume change/surface area. So Fd = PA x ΔV/A = PΔV. (Since P is likely to change as V increases you need to do it as an integral, hence ∫PdV. ) So expansion volume certainly is the determinant, along with pressure, of the available energy.

The expansion on freezing is only about 9% volume, so the work done in expansion when the pressure is released is not that much - enough to bust the container but not much more. There is very little stored energy. You get a lot more stored energy in compressed gases than you do in compressed liquids and solids, because gases expand to many times the confining pressure, doing a lot more work (= energy). That's why heat engines rely on gases.

Aha! Now the rest of your calculation makes sense! I see where you get the deceleration of 2m/sec² from (12m/sec -> 4 m/sec over 4 seconds)and given that the mass is 0.5kg, that will imply a force of 1N, just as you say. So it all looks good to me.

OK. I have however one specific piece of immediate advice for you: learn to use full stops. 😀

These are only "fundamental questions of physics" if the questions have a meaning. And they only have a meaning in physics if they predict some observable result. This is where I struggle. I cannot see what observational outcomes can be dependent on this shape issue of yours. A photon is either detected, by absorption in an atom generally, or they are not. Isn't it?

Where do you get this stuff from about the limits of pumps? And why choose the worst possible type of pump for dealing with big pressure differences? You would need a +ve displacement pump, not a centrifugal one. Injector pumps can manage well over 500bar. And surely by a 2-stage process you could reduce the stresses between each stage considerably, couldn't you?